Study of Calculation Water Surface Evaporation in the “Three Lakes” Basin of Central Yunnan

In order to scientifically and accurately calculate the water surface evaporation in the central Yunnan region， based on the monthly meteorological data from Yuxi Meteorological Station near the 'Three Lakes' basin in Central Yunnan from 2014 to 2021， the effectiveness of the FAO PPP-17 Penman formula in calculating water evaporation in this area and the error sources is analyzed through comparative analysis and correlation analysis.On this basis， aiming at the sources of calculation errors and the characteristics of seasonal changes， the water surface reflectance and the solar radiation parameters in the Penman formula are dynamically modified with the season changes using Morton empirical formula and other methods.Furthermore， based on the principle of heat balance， a new estimation model of water surface heat flux was derived by using error inverse derivation.The water surface heat flux term is added to the FAO PPP-17 Penman formula to correct the heat balance of the water surface.The results showed that： （1） The water evaporation calculated by the FAO PPP-17 Penman formula in the study area was generally overestimated， with a mean relative error of +15.2%.The error has seasonal characteristics of small in winter and large in summer， and periodic variation characteristics which is similar to a sine curve.The calculation error mainly stems from the inappropriate value of solar short-wave radiation parameters in the formula and the neglect of the influence of water surface heat flux， resulting in an imbalance in the heat budget.（2） After dynamically improving the parameters of the FAO PPP-17 Penman formula and adding the water surface heat flux to it， the calculation accuracy is significantly improved.Compared to the unrevised version， the accuracy with relative errors within ± 5%， ± 10%， and ± 20% increased by 40.6%， 42.7%， and 32.3%， respectively.The results of off-site tests show that the optimized Penman equation exhibits high fitting accuracy in the ‘Three Lakes’ basin of central Yunnan， and the model's accuracy with relative errors within ±10% is over 75%.（3） This study developed a nonlinear estimation model for water surface heat flux， which can reasonably simulate the nonlinear response of water storage heat to seasonal variations and water surface radiation intensity.Furthermore， the model has demonstrated satisfactory results in evaporation calculations within the "Three Lakes" region.